The advent of portable computers has created intense demand for display devices which are lightweight, compact and power efficient. Since the space available for the display function of these devices precludes the use of a conventional cathode ray tube (CRT), there has been significant interest in efforts to provide satisfactory flat panel displays having comparable or even superior display characteristics, e.g., brightness, resolution, versatility in display, power consumption, etc. These efforts, while producing flat panel displays that are useful for some applications, have not produced a display that can compare to a conventional CRT.
Currently, liquid crystal displays are used almost universally for laptop and notebook computers. In comparison to a CRT, these displays provide poor contrast, only a limited range of viewing angles is possible, and, in color versions, they consume power at rates which are incompatible with extended battery operation. In addition, color liquid crystal display screens tend to be far more costly than CRT's of equal screen size.
As a result of the drawbacks of liquid crystal display technology, field emission display technology has been receiving increasing attention by industry. Flat panel displays utilizing such technology employ a matrix-addressable array of pointed, thinfilm, cold field emission cathodes in combination with an anode comprising a phosphor-luminescent screen. The phenomenon of field emission was discovered in the 1950's, and extensive research by many individuals, such as Charles A. Spindt of SRI International, has improved the technology to the extent that its prospects for use in the manufacture of inexpensive, low-power, high-resolution, high-contrast, fullcolor flat displays appear to be promising.
Advances in field emission display technology are disclosed in U.S. Pat. No. 3,755,704, "Field Emission Cathode Structures and Devices Utilizing Such Structures," issued 28 Aug. 1973, to C.A. Spindt et al.; U.S. Pat. No. 4,940,916, "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodoluminescence Excited by Field Emission Using Said Source," issued 10 Jul. 1990 to Michel Borel et al.; U.S. Pat. No. 5,194,780, "Electron Source with Microtip Emissive Cathodes," issued 16 Mar. 1993 to Robert Meyer; and U.S. Pat. No. 5,225,820, "Microtip Trichromatic Fluorescent Screen," issued 6 Jul. 1993, to Jean-Frederic Clerc. These patents are incorporated by reference into the present application.
The Clerc ('820) patent discloses a trichromatic field emission flat panel display having a first substrate on which are arranged a matrix of conductors. In one direction of the matrix, conductive columns comprising the cathode electrode support the microtips. In the other direction, above the column conductors, are perforated conductive rows comprising the gate electrode. The row and column conductors are separated by an insulating layer having apertures permitting the passage of the microtips, each intersection of a row and column corresponding to a pixel.
On a second substrate facing the first, the display has regularly spaced, parallel conductive stripes comprising the anode electrode. These stripes are alternately covered by a first material luminescing in the red, a second material luminescing in the green, and a third material luminescing in the blue, the conductive stripes covered by the same luminescent material being electrically interconnected.
The Clerc patent discloses a process for addressing a trichromatic field emission flat panel display. The process consists of successively raising each set of interconnected anode stripes periodically to a first potential which is sufficient to attract the electrons emitted by the microtips of the cathode conductors corresponding to the pixels which are to be illuminated or "switched on" in the color of the selected anode stripes. Those anode stripes which are not being selected are set to a potential such that the electrons emitted by the microtips are repelled or have an energy level below the threshold cathodoluminescence energy level of the luminescent materials covering those unselected anodes.
An example given in the Clerc patent recites voltages on the anode electrodes for attracting emitted electrons in the range of 100-150 volts, with the voltage on the unselected anode electrodes at 40 volts. Recent experimentation, however, has indicated that substantially higher attracting voltages, in the range of 500-800 volts, are required to provide a satisfactory display, while the voltage on the unselected anode electrodes must be substantially zero for the desired purity of color. Since the attracting voltage on each anode electrode is switched on for a color field (or subframe) period of 5.56 milliseconds in each frame period of 16.67 milliseconds, for an illustrative frame rate of sixty frames per second, the switching losses for a several-hundred-volt swing at that rate are substantial. Where the field emission display device is used in a portable, battery-operated system, such as a notebook computer, large switching losses are incompatible with a desired goal of extended battery life.
In view of the above, it is clear that there exists a need for an apparatus for reducing the switching losses in a switched anode, field emission display, while permitting the use of anode voltages in the range of 500-800 volts, and switching the voltage on the unselected anode electrodes to substantially zero.